Whether we want to or not, many folks have to interact with CMake at least once in their life. If they are unlucky, they might have to find dependencies using find_package. If they are extremely unlucky, they might have to write a file to work with find_package. At the end of this, they might have a file that kind of works, but most likely is based off of a tutorial from 2008. A lot of things have changed since then. There's an easier way to do it these days!
In a previous post, I alluded to a project I've been working on since 2018. It's called IXM, and while it's not ready for general use, it is where I realized there is a common set of operations for finding packages, their components, and setting their correct properties. We'll be using some of what I've learned and figured out over the past two years to understand how find_package files work and how to make them useful for your projects and the folks who depend on them.
This tutorial is written for CMake 3.14 and later, though I personally recommend using CMake 3.16 as it is the version installed with the most recent Ubuntu LTS release, 20.04.
Additionally, this tutorial isn't meant to discuss how to write a <package-name>-config.cmake file. Those have a different set of options but also tend to be smaller in practice. Instead I'll be showing how to write what's known as a find_package MODULE file. That said, we will eventually tackle how to handle writing a usable find_package file that can be used in cmake --find-package mode, in the so-called Script Mode or with cpack for the External Generator. For now, however, we'll focus on the most common workflow for CMake: Configure and Build.
The Basics
Within CMake, there are several commands that are used when writing a find_package file. The most important ones are find_program, find_library, find_path, and lastly find_file. Each of these has a purpose, but we will not always use them. Depending on what you are trying to find, you might also find yourself using execute_process, file(READ), file(STRINGS), string(REGEX MATCH), and mark_as_advanced. Additionally, 99.9% of the time, you'll want to use the CMake provided module FindPackageHandleStandardArgs.
Before we can use find_package, however, we need to make sure CMake can find it in the first place. The most common place to put your cmake scripts is inside the project's root directory under a cmake/ directory. We can then add this path to our CMAKE_MODULE_PATH variable so CMake knows where to find us.
This is the bare minimum we need for CMake to find our following find_package files.
Understanding the Commands
The commands most typically used have a very large amount of documentation and thus it can be a bit overwhelming. Worry not. Most of the time you don't need to worry about these additional parameters. We can cover them in detail in later posts.
find_program
This is used to find the path to a file that the system considers executable. Whether it is a script or an actual executable is actually irrelevant. As long as you could execute it via something like exec (or CreateProcess on Windows), it can be used as an executable.
find_library
This is used to find both shared and static libraries. The form for this is a bit odd, as languages with custom static library formats (e.g., Rust's .rlib) won't ever be found, however we can find .so, .dylib, .dll, .a, and .lib files by default. on macOS, .frameworks are also searched for first. Users can configure this with the CMAKE_FIND_FRAMEWORK variable.
Finding an Executable
Finding executables in packages is one of the easiest things, compared to finding libraries. Executables can be made into imported targets, much like libraries, and they can then be used in the add_custom_command and add_custom_target commands with their imported name.
A popular tool that we can use as an example here is sphinx-build, the actual "compiler" for the sphinx documentation framework.
Inside of a file named FindSphinx.cmake, we have
You'll notice that, unlike what you might be used to, we use Sphinx_EXECUTABLE and not SPHINX_EXECUTABLE. This will make more even more sense in later posts, but it's consider "good behavior" to prefix your variables with the package name, and not an upper cased version of said package name. This also prevents a theoretical find_package(SPHINX) and a find_package(Sphinx) having a variable collision.
That's the basics of it! We can now let find_package_handle_standard_args take care of business for us, and then hide our cache variable if it's been found.
Seems simple enough, right? Well, we still need to import this executable so it's usable by the build system. To do that we're going to create what's known as an imported executable and set the IMPORTED_LOCATION property to the value stored in Sphinx_EXECUTABLE
Of course, it's not enough to just do this. We need to guard our sphinx target creation. What if we never found it in the first place? The file would error for users and that's no bueno!
Additionally, what if someone wants to name their target Sphinx? It's best to try to stay out of their way, or our simple documentation generator might interfere with someone's actual dependency or project! We can do this by namespacing the Sphinx target, which is only permitted for imported targets.
But wait! What if multiple projects in our build tree depend on find_package(Sphinx)? Then it will fail to work because Sphinx::Sphinx is already a target! We should fix that by only creating the target if we've found Sphinx and it hasn't been created yet.
While this might seem simple, once we move into later posts in this tutorial, we will expand on this and get it to its proper state.
Finding a Library
Finding libraries in CMake is deceptively simple. It should just be as easy as finding the path to a single file, setting a variable, and calling it a day... right?
If only that were the case. Remember, we have access to imported targets, and this can be used to provide more information to CMake when generating the build system, and prevents typos for variables. Even more important, we can create dependency lists of libraries and their components so that users can follow the "YOLO" principle (i.e., You Only Link Once).
Worse, sometimes these libraries provide <Library>Config.cmake/<library>-config.cmake files which might not create imported targets and simply export cache variables. One such library is SDL2. SDL2 is a wonderfully neat library, but it's CMake experience leaves a lot to be desired. Sounds like a perfect way to create our own FindSDL2.cmake file.
Using what we've learned above regarding find_package for executable based packages, we can start with a basic skeleton file that's eerily similar to what we did before.
Try to use this however, and you'll quickly find there is no way to #include <SDL2/SDL.h> 😱! That just won't do! We need to find the path where this file is found. This is where find_path comes into play to find the SDL2_INCLUDE_DIR. It's a requirement for us to use SDL2 at all, so it'll be added to the REQUIRED_VARS parameter to find_package_handle_standard_args. Because it's a cache variable, we'll also mark it as ADVANCED if we found SDL2, and finally add it to the INTERFACE_INCLUDE_DIRECTORIES property for SDL2::SDL2
Awesome! We can now just call target_link_libraries(<my-target> PRIVATE SDL2::SDL2) and we've got full access to its headers and automatically link against the library itself.
That... should be it right? Unfortunately, no. On Windows, we must sometimes link against SDL2main (other platforms are permitted to as well), because SDL2 sometimes overrides the main function with a redefinition. We can ask that find_package search for SDL2main by declaring a component called SDL2::Main. Ideally, we will always search for all components, and let find_package and find_package_handle_standard_args take care of the details. However we need to (of course) do some extra work.
Firstly, we need to find the SDL2main library itself. CMake's find_package cares less about how variables are named, but does care about how the _FOUND variables are named. Effectively, for each component in a package, find_package_handle_standard_args considers a component found if <package>_<component>_FOUND is true or false. Generally, you'll want to stick to a consistent naming convention, so we'll name our _LIBRARY variable SDL2_Main_LIBRARY, and if it's set at all, we'll just set SDL2_Main_FOUND to YES (one of the strings CMake considers to be a boolean)
We then pass HANDLE_COMPONENTS as a flag to find_package_handle_standard_args.
Next we'll need to also mark the variable as advanced but only if its found and ONLY if find_package_handle_standard_args didn't return early. This might seem counter-intuitive, but that's CMake for ya! We might as well also import the library and make it depend on SDL2::SDL2.
That should be it, right? Again, things are not as simple as they may seem. SDL2 introduces bug fixes, new APIs, etc. in minor point releases. How do we know for sure we're building with SDL2 2.15? We need to check the version. How does SDL2 express this? Inside of the SDL2/SDL_version.h header. This, here, is the part where things will become very painful. The only solution we have is to... use a regex 😱😭.
SDL2 has the following C preprocessor defines declared:
SDL_MAJOR_VERSIONSDL_MINOR_VERSIONSDL_PATCHLEVEL
Luckily, these are fairly simple to find, so we can just get a list of matching lines via file(STRINGS)
OK, but that doesn't actually get us the data right? So we need to extract it further, and this is where it really does get messy. We'll also pass it to find_package_handle_standard_args as the VERSION_VAR and then set the VERSION property of SDL2::SDL2
And that's all we need to do! We're done 🎉 (for now 😱)
Exhausting wasn't it? Luckily, once you write a file like these you rarely need to ever touch them again... At least, until the next entry in this series. 😈
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